1. Field of the Invention
The present invention relates to an interferometric confocal method, an interferometric confocal assembly for optical data storage devices, in particular terabyte volume storage devices, and a use of a data storage device.
2. Description of the Related Art
The read-out of data from a DVD can still take place at an excessively low speed even in modern real-time applications. For example if the intention is for simultaneous replay of a plurality of feature films with high picture quality from a single DVD. Moreover, there are increasingly applications where even the storage volume of a 17 Gbyte DVD is nowhere near adequate, for example in video archiving or data backup in the civil service and in databases where a very high data transfer rate is required.
The confocal technology, also in combination with interferometry, is well suited to the read-out of optical volume storage devices with multi-layers, also see Stephen R. Chinn and Eric A. Swanson: “Optical Coherence Tomography for High-Density Data Storage” in Handbook of Optical Coherence Tomography, Editors: Bouma, Brett, E.; Tearney, Guillermo, J.; Marcel Dekker, Inc., New York, Basel, 2002, ISBN number 0-8247-0558-0, [1] chapter 14, pp. 385-420. These principles are presented in U.S. Pat. No. 5,784,352 by Eric A. Swanson and Stephen R. Chinn.
The read-out of data by means of confocal technology can be effected in diffraction-limited fashion to a very good approximation. The confocal technology thus represents a very exploitable approach for further increasing the storage volume of optical volume storage devices. Problems with the signal quality can occur in the case of a very large number of layers arranged one above another, however, as a result of the scattering in the volume, particularly when reading from the deeper layers. There is an appreciable advantage with regard to sensitivity if the confocal technology is combined with an interferometric approach, that is to say that a confocal interferometric method is used, also see [1], p. 409. This advantage over the purely confocal technology also generally holds true for the detection of scattering centers in a larger depth in an unstructured transparent storage medium.
A further problem in the multi-layer technology is the spherical aberration in the storage medium, which greatly limits the usable depth region in the storage volume in the case of high-aperture fixed objectives; in this respect, also see [1], p. 408. Here it is explained that a fixed objective having a numerical aperture of 0.55 at a wavelength of 780 nm, despite post-focusing, owing to the spherical aberration, can only be used in a depth region of about +/−100 μm around the average focus position of 1.2 mm in the case of a standard polycarbonate disk without the occurrence of signal degradation as a result of wavefront deformations. This means that, owing to the influence of the spherical aberration, for example only about 10 layers can be read with good signal quality. In this case, with regard to the sensitivity, a confocal interferometric method—referred to as OCT system in [1], p. 409—could certainly read from 100 layers in the depth given suitable embodiment of the disk medium and layer configuration. However, this then requires an active optical arrangement for compensating for the spherical aberration, said active optical arrangement also being known as dynamic optical compensation. Said active optical arrangement increases the complexity of the scanning system to a very great extent, however, and considerably impairs the temporal dynamic characteristic of said system.
Moreover, the known approach of optical data read-out by means of confocal interferometric methods, also see [1], pp. 386-389, also referred to here as “optical coherence domain reflectometry”, or OCT technology, only ever enables the read-out of a single layer or data track at one point in time, namely the read-out of the layer onto which the electromagnetic radiation is currently being focused for the read-out. Thus, it is not possible for a plurality of data tracks to be read simultaneously at one point in time and, consequently, the read-out speed is indeed very low in view of the very large quantity of data of a multi-layer data carrier that effects storage virtually in diffraction-limited fashion laterally.
White light interferometry with spectral evaluation, also known as Fourier domain OCT or spectral interferometry, also see M. W. Lindner, P. Andretzky, F. Kiesewetter and G. Häusler: Spectral Radar: Optical Coherence tomography in the Fourier Domain in [1], pp. 335-34.5, is likewise much better suited than the exclusively confocal technology to reading out a larger number of storage layers in the depth, owing to the comparatively good dynamic characteristic of the detectable signals. The problem, with regard to the read-out of optical volume storage devices by means of spectral interferometry, is the comparatively small numerical aperture of the focusing objective. Upon application of spectral interferometry for the optical data read-out from a storage medium, this would lead to laterally comparatively large dots or pits in the storage medium and hence to a significantly lower storage density per layer in comparison with the standard technology. This means that the economic benefit of this known approach for the optical data read-out is rather limited.
Independently of this, it has also already been proposed to read from a larger number of storage layers simultaneously in the depth and also laterally from a storage volume virtually in diffraction-limited fashion, solely by means of high-aperture confocal technology. However, purely confocal technology falls far short of achieving the sensitivity of the confocal interferometric methods, which, according to [1], p. 409, is two orders of magnitude better than that of the purely confocal technology.
Approaches for chromatic confocal microscopy have already been presented by H. J. Tiziani and H.-M. Uhde in the specialist article Three-dimensional image sensing by chromatic confocal microscopy in Applied Optics, Vol. 33, No. 1. April 1994, pp. 1838 to 1843. This approach enables the mechanical depth scan to be dispensed with. The read-out of optical data carriers was not of primary significance in these applications.
The publication “Accurate fiber-optic sensor for measurement of the distance based on white-light interferometry with dispersion” by Pavel Pavlicek and Gerd Häusler in ICO Tokyo, paper no. 15B3-1 on Jul. 15, 2004 [12], describes an assembly in which a signal that is intensity-modulated by way of the wave number is generated in a fiber in the reference arm of an interferometer by means of dispersion. However, here as well the object distance can only be determined within the physical-optical depth of focus of the sensor head, which is determined by the numerical aperture of the objective of said sensor head, and is therefore highly limited in particular for a high numerical aperture.
K. Körner, P. Lehmann, A. Ruprecht and W. Osten have also proposed a chromatic confocal assembly for reading optical data carriers. However, said assembly still has potential for improvement with regard to the sensitivity of the assembly.
Although holographic methods permit stored information to be read out from a volume, they generally have a non-diffraction-limited storage density and are therefore not considered any further here, also see [1], p. 386.
The aim of the invention is to make it possible, for commercial use, to read comparatively rapidly optical multi-layer data carriers, that is to say volume data carriers that are transparent in the base material with a very high storage density and storage capacity, for example also in the terabytes range. The aim in this case is to read from volume data carriers with a lateral storage density as near as possible to the limit due to the diffraction of electromagnetic waves and for the use of a focusing optical arrangement having the highest possible aperture, for example with a numerical aperture of at least 0.5. Moreover, the intention is to make it possible to obtain a comparatively high storage density in the vertical direction by virtue of the data layers lying comparatively close together as a result of the application of the invention. This is intended to become possible through the use of a focusing optical arrangement with a high numerical aperture. In this case, the intention is to make it possible to achieve a dataflow that is as large as possible by means of the inventive read-out method when reading from the volume data carrier, for example for simultaneous replay of a plurality of feature films with very high picture quality.
The aim of the invention is furthermore to achieve a comparatively high robustness toward vibrations during the read-out operation and smaller manufacturing faults of the data carrier. The aim is furthermore to dispense with an active optical arrangement, at least in the read-out operation, in particular in order to achieve a high read-out speed and comparatively low production costs and also a long product service life.
A further aim is also to be able to read data carriers which have a very good long-term stability of the data storage. These data carriers are then likely to have a lower storage capacity. Therefore, the intention is also to make it possible to apply the method also to storage materials which, in the event of wear, destruction or weathering of the surface of the optical data carrier, make it possible as necessary to repolish the optical contact area thereof. Upon application of the invention the intention is for example to make it possible also to read dots “burnt” by means of femtosecond lasers, in different depths of the storage volume, for example also dots in high-strength inorganic and transparent materials.
Thus, the inventive object to be achieved is therefore in particular that of simultaneously detecting optical features very rapidly and with high reliability when reading out data from an optical data storage device also in significantly more than two layers in the depth or in volume regions of different depths.
Furthermore, the inventive object to be achieved is therefore also that of achieving a high robustness in the read-out operation by virtue of the fact that the read-out system, that is to say the optical sensing head, can still supply data comparatively reliably at the point in time of the read-out even when it is in a certain incorrect position in the depth.